fix: harden IMDb IRC output

[variablenix]

Summary

• enforce the 512-byte IRC wire limit with UTF-8-safe truncation on every IMDb response path
• remove variation selectors, invisible Unicode format controls, IRC controls, and line breaks from IMDb output
• reject unsafe invisible characters in queries
• add regression coverage for one-message behavior, multibyte output, upstream failures, malformed JSON, and cancellation
• document byte-limit and terminal-safety behavior

Validation

• go test -count=1 ./...
• go test -race -count=1 ./...
• go vet ./...
• go mod verify
• govulncheck ./... (no vulnerabilities found)